Long-term modifications in synaptic efficacy are implicated in learning and memory; dysfunction of this plasticity is implicated in a variety of neurodevelopmental and neuropsychiatric disorders. The long-term goal of this project is to elucidate the supramolecular organization of the postsynaptic density (PSD), which plays a central role in synaptic signal processing. This information may ultimately prove very useful in designing novel approaches to the prevention or treatment of brain disorders. The proposal for this funding cycle includes three specific aims: Aim 1 is to develop new electron microscopic tools to study the ultrastructure of synapses, and to use these tools to study abnormalities of the PSD in mouse models of autism and schizophrenia. Aim 2 is to examine how actin filaments attach to the PSD, to study the organization of two isoforms of a protein that links actin to the PSD, and to study two PSD-associated enzymes that control Rho-family proteins (molecular switches that modulate actin remodeling). Aim 3 is to study the alignment of presynaptic release sites with postsynaptic receptors, and to explore possible disruptions in synaptic structure and receptor expression in mice that have mutations in synaptic adhesion molecules.